1. Field of the Invention
The present invention concerns the field of electrochemical generators, and more particularly the field of accumulators.
It specifically relates to generators having a zinc anode and aims to provide the zinc electrode with a high recycling capability.
2. Description of the Prior Art
Because of its high performance the zinc electrode has long been a subject of interest for persons skilled in the art, and has been used in different types of secondary electrochemical systems: zinc-air, nickel-zinc, silver-zinc, bromium-zinc and chlorine-zinc generators.
Zinc constitutes a particularly attractive active electrode material having a strongly negative redox potential. The zinc electrode has a theoretical capacity per mass of 820 Ah/Kg and enables theoretical power per mass ratios of 334 Wh/Kg for the nickel-zinc couple, and 1,320 Wh/Kg for the zinc-oxygen couple. For Nixe2x80x94Zn accumulators, the practical power per mass is usually situated between about 15 and 80 Wh/Kg, which is comparable to that of the Pbxe2x80x94PbO2 couple, which is of the order of 25 to 30 Wh/kg.
It should also be emphasized that the advantages of zinc include, on the one hand, its property of non-toxicity for the environment (manufacturing, use, waste), on the other hand, its low cost, much less than that of other anodic materials for alkaline accumulators (cadmium and metallic hydrides).
However, still in the relation to alkaline accumulators, the industrial development of rechargeable systems using a zinc electrode has encountered one major difficulty, the insufficient electrode life-time when cycled.
Hence, in secondary systems having an alkaline electrolyte, the formation of deposits whose structure is modified relative to the original morphology, most usually dendrites or powdery deposits, leads to recharging the zinc electrode from its oxides and hydroxides and zincates. This phenomenon furthermore takes place throughout a large range of current densities.
Dendridic-type deposits rapidly lead to growth of zinc protuberances through the separators and to short-circuiting with electrodes of opposite polarity, for example with a nickel cathode in the case of Nixe2x80x94Zn.
As for powder-type deposits, they most usually are not of such nature as to permit the reconstitution of electrodes capable of satisfactory operation, because the adherence of the active material on the support is insufficient.
Furthermore, the reduction of oxides, hydroxides and zincates to zinc on the anode, during recharging, is also characterized by changes of morphology of the said electrode. Depending upon the modes of operation of the accumulators, different types of modifications in the anode shape are observed, by a phenomenon of non-uniform redistribution of the zinc during its formation, which can in particular manifest itself by an undesirable densification of the active anode mass.
These severe handicaps reduce the number of cycles that can be carried out down to only several tens of cycles, which is insufficient to make the systems economically viable. This has lead to a great deal of work aiming to improve the characteristics of the deposit when recharging, with a view to increasing the number of charging-discharging cycles.
Extremely diverse ways have been explored to delay as long as possible both the dendridic growth and the non-uniform redistribution of the zinc. Amongst these ways, the following in particular should be noted:
the addition of additives incorporated in the electrolyte or in the active anode material, notably intended to limit the solubility of zincates;
the use of mechanical processes aiming to reduce the formation of dendrites and to avoid powdery deposits (circulation of the electrolyte and/or of the zinc electrode in a dispersed phase);
control of the charging parameters (current, voltage, . . . ); the use of pulsed currents; or
to delay the consequences of the dendridic growth, also the use of separators resisting the formation of dendrites, notably microporous separators or even exchange membranes.
These various techniques can be implemented singly or in combination.
The positive effects thereof are in any event limited, and have proven to be insufficient to make zinc-electrode secondary generators, in particular the Nixe2x80x94Zn couple which however is theoretically very attractive, approach economic viability.
Certain of these techniques additionally have penalizing negative effects: increase of the internal resistance of the accumulator, degradation of the cathode life-time, notably when using certain additives; also the mechanical complexity of operation, increase of the volume, the mass, and cost.
As a result of the growing need for portable energy sources and rechargeable systems, for supplying portable electric or electronic apparatus, for supplying the increased amount of electronic equipment in automobiles, or for the propulsion of electric or hybrid vehicles, ways for simultaneously meeting up to the following quality criteria ought to be made available:
high performances per mass and per volume;
low cost, compared to other types of existing secondary alkaline systems;
absence of toxicity.
The aim of the present invention is to provide a new, original and satisfactory solution for recharging the zinc-electrode by the production of a homogenous and non-dendritic deposit which enables several hundreds of cycles to be carried out through a large range of operating conditions.
The characteristics obtained result from the means of implementation whose purpose is to increase the utilization of active material by improving percolation of the electrical charges, and increasing the charge-discharge efficiency.